The present invention relates to the electrical control arts. It finds particular application in conjunction with the precise control of tube currents in x-ray and other vacuum tubes and will be described with reference thereto. The invention finds particular application in controlling the x-ray tubes of medical diagnostic devices, such as CT scanners which require precise adherence to narrow tolerances.
Conventionally, an x-ray tube includes a thermionic filament cathode and a rotating anode which are encased in an evacuated envelope. A heating current, commonly on the order of 2-5 amps is applied through the filament to create an electron cloud therearound. A high potential, e.g. 50-150 kilovolts, is applied between the filament and the anode to accelerate the electrons from the cloud to an anode target area. This acceleration of electrons causes a tube or anode current which is commonly on the order of 5-200 milliamps. The tube current and the x-ray emitted from the anode vary with both the high potential across the tube and the temperature of the filament. The filament temperature, in turn, varies with the filament current, voltage, and internal resistance.
In CT scanners, one of a plurality of preselected voltages is applied across the anode and cathode by conventional power supply circuitry. To control the filament temperature, U.S. Pat. No. 4,311,913, issued Jan. 19, 1982 to the inventors herein, utilized a feedback loop which adjusted the filament voltage as a function of the deviation, if any, between the actual tube current and a preselected tube current. In preparation for a scan and between scans, a small stand-by filament current was applied. When a scan was to commence a high voltage was applied across the tube followed about 18 milliseconds later by closing a feedback loop to regulate the heating voltage applied to the filament as a function of the tube current. Due to cable and contact resistance, there were deviations between the regulated filament power supply output voltage and the actual filament temperature and voltage. These deviations caused inconsistency in the regulation of the filament temperature. After the scan was completed, the high tube voltage was removed and the filament current was returned to the lower stand-by current after a short post heat period.
In accordance with the present invention, a faster, more accurate x-ray tube control circuit is provided.